1. Field of the Invention
The present invention relates to a rubbing machine for use in a liquid crystal display manufacturing process. More particularly, the present invention relates to a rubbing machine for use in a liquid crystal display manufacturing process, and also to a rubbing method which is conducted using the same, in which the rubbing machine has a realigning function of a rubbing cloth and is suitable for use in a rubbing process involved in the manufacture of a liquid crystal display panel and for use in the fabrication of equipment for manufacturing a liquid crystal display.
2. Description of the Prior Art
A rubbing process, in which an alignment film coated on a substrate is rubbed with a cloth and the like to determine the direction of alignment, is a critical process in an LCD manufacturing process. Recently, in addition to twisted nematic (TN) mode, various modes including in-plane switching mode, fringe field switching (FFS) mode, and optical compensated bend (OCB) mode were proposed and applied to products.
A common point between these modes is that rubbing directions of upper and lower substrates is parallel to each other (0° or 180°). This parallel rubbing is not a special case. Actually, the case where the rubbing directions of the upper and lower substrates are parallel to each other is rather predominating, and the TN mode where the rubbing directions of the substrates are 90° may be regarded as a special case.
In the case of the parallel rubbing, rubbing uniformity is a very important factor that is linked directly with image quality. Since IPS mode and FFS mode are designed as a normally black mode, images in a dark state are determined by the rubbing uniformity.
However, as shown in FIG. 1, it is difficult to achieve uniform rubbing conditions in the mass production concept, so that poor image quality significantly occurs due to rubbing non-uniformity.
In the mass production concept, the reason why the rubbing non-uniformity occurs is as follows.
A rubbing cloth is generally formed of a velvet-shaped cloth where short and thin piles are implanted on a base cloth. Also, the rubbing cloth is mainly made of cotton or rayon.
Rubbing of the alignment film is achieved by the piles implanted on the base cloth, which come in actual contact with the alignment film.
These piles have elastic force and restoring force so that they rub the substrate while exhibiting some buffer action even on the non-uniform portions of the alignment film and the substrate. However, in the mass production concept, there are large amounts of work that one rubbing cloth will perform. Thus, if non-uniformity at a fixed position is continued, it then exceeds the buffer limit of the rubbing cloth and affects the rubbing cloth, so that the surface of the rubbing cloth corresponding to that portion will be non-uniform.
Accordingly, as the rubbing working progresses, rubbing of the substrates that are rubbed later becomes increasingly non-uniform. Thus, in the case of the parallel rubbing, the amount of work conducted by one rubbing cloth is significantly limited as compared to TN mode.
Meanwhile, there are innumerable non-uniform factors that can occur during a process. These factor include non-uniformity of a rubbing state, glass edges, TFT patterns and lines formed in an array, pixels and black matrixes of a color filter, and column spacers. In this case, a problem is that they are factors necessary for LCD panels. Thus, eliminating such factors to solve the rubbing non-uniformity is not possible.
Meanwhile, once a new rubbing cloth is attached on a rubbing roll, it is subjected to an aligning step of rubbing several times a test substrate (dummy substrate) such as a glass substrate, before it is applied for a mass production process. This aligning step is generally called “aging technique”, i.e., a realignment technique of the rubbing cloth. It was found that this aging technique was effective even in realigning the rubbing cloths having non-uniformity occurred at the start and middle of a process.
Moreover, this realignment technique of the rubbing cloth is highly effective in lengthening the life span of the rubbing cloth and in maintaining higher process quality.
As shown in FIG. 2, the realignment technique of the rubbing cloth is one in which several or several tens of mass production substrates A are rubbed and one or several sheets of realigning substrates, such as a glass substrate having a flat surface, are then rubbed.
By these realigning substrates inserted between the mass production substrates contributing to non-uniformity, the rubbing cloth whose non-uniformity have been deepened is reduced to an initial condition of a process.
Furthermore, in applying this realignment technique of the rubbing cloth, inserting the realigning substrates B between the mass production substrates A is a troublesome operation, and the realigning substrates B are mixed with the mass production substrates A so that poor image quality can be caused.
As a result, as long as a rubbing machine has a function capable of realigning the rubbing cloth by itself without separately inserting the realigning substrates B, the problem of causing the poor image quality can be solved.